The advent of combined antiretroviral therapy (cART) has substantially extended the lifespan of HIV-1 patients; yet, the prevalence of HIV-1 associated neurocognitive disorders (HAND), which can be exacerbated by co-morbid opioid drug abuse, overall has not changed. Many HAND symptoms are thought to be caused by the selective degeneration of particular brain regions, such as the striatum, during HIV-1 infection. The striatum is comprised of approximately 90-95% medium spiny neurons (MSNs). The MSN population is heterogeneous, and can be divided in subtypes based on their expression of different combinations of surface markers and proteins, such as dopamine 1 and 2 (D1, D2), adenosine A2A, and muscarinic M4 receptors, as well as, substance P, dynorphin, and enkephalin, with D1- and D2-expressing MSN as the main subtypes. Assessment of MSN subtype vulnerability to HIV-1/opioid-use could enhance our understanding of how HIV-1 and co-morbid opioid use lead to HAND symptoms, and suggest refined preventative and functional treatments. The proposed studies will take advantage of dopamine Drd1 (D1) or Drd2 (D2) -receptor promoter-driven tdTomato- or EFGP-expressing mouse lines, respectively, to identify D1- and D2-receptor- expressing MSNs. These mouse lines, along with wild-type controls, will be used to investigate lethal and sub-lethal effects of the HIV-1 protein transactivator of transcription (Tat) and opioids on the subtypes of MSNs using both in vitro and in vivo models. Primary mixed cortical-striatal, neuron-glia cultures will be employed to determine the selective direct and glial-induced indirect neurotoxic and synaptodendritic degenerative effects of Tat and morphine on these subtypes. We will then use a D1-tdTomato- and D2-EGFP-expressing mouse line crossed with a doxycycline-inducible Tat-expressing mouse line to explore if previously noted Tat- and/or morphine-induced motor disturbances are selectively mediated through synaptodendritic degeneration, and/or electrophysiological changes in a particular subset of MSNs. This will be accomplished using a battery of mouse-motor assays, whole-cell patch clamp, and three-dimensional reconstruction of D1- and D2-expressing MSNs. Taken together, these experimental results will show if a subgroup of MSNs is driving the previously noted Tat- and morphine-induced lethal and synaptodendritic degenerative effects, and their correlation with motor impairments. The combination of in vitro approaches, which allow for precise control of experimental conditions, and in vivo tests, which will show a functional/morphological correlation, will provide the first described insights into the selective vulnerability of MSN subtypes to HIV-1 and/or opioids.